1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to a method for improving linearity of touch system coordinates.
2. Description of Related Art
The operation principle of touch panels is to sense a voltage, a current, an acoustic wave or an infrared when a finger or other medium touches on a touch screen, so as to detect the coordinates of touching points. For example, a resistive touch panel uses the voltage difference between upper and lower electrodes to calculate the location where a force is applied, to thereby detect the touching point. A capacitive touch panel uses the current or the voltage originated from capacitance changes in a static electricity combination of transparent electrodes in row and column with human body to detect the touching coordinate.
Mutual capacitance sensing scheme is known as projected capacitive sensing techniques. When there is a touch occurred, the mutual capacitance sensing indicates that a capacitance coupling is generated between two adjacent conductor lines.
The way of driving the mutual capacitive touch panel is to sense a magnitude change of mutual capacitance Cm, so as to determine whether the object is approached to the touch panel. The mutual capacitance Cm is not a physical capacitor but a mutual capacitance between the conductor line in a first direction and the conductor line in a second direction. FIG. 1 is a schematic diagram of a typical mutual capacitance sensing. As shown in FIG. 1, the drivers 110 are arranged on the first direction (Y), and the sensors 120 are arranged on the second direction (X). At the upper half of the first period of time T1, the drivers 110 drive the conductor lines 130 in the first direction and use the voltage Vy_1 to charge the mutual capacitance (Cm) 140. At the lower half of the first period of time T1, all sensors 120 sense voltages (Vo_1, Vo_2, . . . , Vo_n) on the conductor lines 150 in the second direction so as to obtain n data. Accordingly, m*n data can be obtained after m driving periods.
Such a mutual capacitance sensing has the advantages as follows:
(1) It is easily determined whether a touch is generated from a human body since a signal generated from a floating conductor is in a different direction than a grounded conductor.
(2) Every touch point is indicated by a real coordinate, and the real position of each point can be found when multiple points are concurrently touched, so that such a mutual capacitance sensing can easily support the multi-touch applications.
Also, there are some disadvantages as follows:
(1) A single image raw data has an amount of n×m, which is relatively large.
(2) A one-by-one scanning is operated in a selected direction. For example, when there are 20 conductor lines in the first direction (Y), the sensing operation is performed 20 times for obtaining a complete image raw data. Also, due to the large amount of data, the time required for converting a sensed signal from analog into digit is relatively increased.
(3) Due to the large amount of data, the power consumption is thus increased on data processing.
The mutual capacitance sensing technique is typically used in a specific capacitive touch panel. By two-dimensional (X direction and Y directional) driving and sensing, it is able to obtain two-dimensional voltage change related to capacitors from the capacitive touch panel. The voltage change is known as raw data, and the raw data may include variation caused by the influences of touch and environmental noise.
The raw data has to be processed by certain algorithm or interpolation method so as to compute the touch coordinate, thereby determining the position of the touch panel at which the user touches. In the prior art, for processing the raw data, a threshold is typically configured to determine whether there is effective touch data.
As shown in FIG. 2, it determines the touch area based on whether the raw data is greater than the threshold. That is, all the raw data with values greater than 80 are determined to be a touch area.
However, such a prior art may cause a serious problem, i.e., the linearity of the raw data and whether the sensing value is excellent or not are greatly influenced by the electrode pattern of the capacitive touch panel. FIG. 3 schematically illustrates the non-linearity displayed in the prior art, which uses a general electrode pattern in cooperation with the threshold determination method. From FIG. 3, it is known that the linearity of such a prior method is poor.
In addition, the electrode pattern design of the capacitive touch panel has a great influence to the values of the raw data. FIG. 4(A) to FIG. 4(C) schematically illustrate how the electrode pattern influences the values of the raw data, in which the slash area stands for a user touch area. When the touch area moves from right to left, it is obvious to have relatively large sensing value when the sensing lines (such as S1-S5 in the figure) in the vertical direction are touched. When there is no touch, the corresponding value is greatly reduced. Therefore, whenever a finger goes over the sensing lines, the values of raw data are dramatically changed, and thus it is known that the values of the raw data are greatly influenced by the electrode pattern. If the threshold determination is further employed to determine whether there is effective touch data and thus to compute the touch coordinate in the prior, it is likely to cause a ladder-like touch coordinate non-linearity exhibition as shown in FIG. 3.
FIG. 5 shows the data actually measured by the prior art, which schematically illustrates the track of touch points obtained when a finger or a mechanic object slightly contacts and crosses a touch panel. The horizontal axis and vertical axis respectively represents the corresponding positions of the touch panel. As shown in FIG. 5, the prior art may cause a ladder-like non-linearity exhibition when the touch position passes through the sensing lines.
To solve the problem, a direct approach is employed to reduce the threshold for increasing the linearity. However, such an approach may cause the generation of noise points.
FIG. 6(A) to FIG. 6(D) schematically illustrate the process with reduced threshold, wherein FIG. 6(A) shows a touch position of a finger, FIG. 6(B) shows the values of the obtained raw data, FIG. 6(C) shows the slash part that is determined to be touch position when the threshold is set to be 80, and FIG. 6(D) shows the slash part that is determined to be touch position when the threshold is reduced to be 30, while the number of noise points is increased.
That is, the design of threshold is very difficult. If the threshold is too high, it will be hard to determine the touch control, resulting in unsatisfactory linearity in subsequent coordinate conversion. On the other hand, if the threshold is too low, the linearity exhibition can be preserved but it is likely to be influenced by noise interference, resulting in the occurrence of many unpredicted coordinate points in addition to the specific touch position.
FIG. 7 schematically illustrates a continuous observation of raw data of a certain pixel at an intersection of X-axis and Y-axis on a touch panel for a period of time, in which the horizontal axis is time axis and the vertical axis is raw data and output data. A threshold is defined to determine whether there is touch data. If the threshold is defined to be smaller (as indicated by the solid line), raw data of higher linearity can be preserved but it is likely to be influenced by noises (as indicated in the rectangular box), resulting in occurrence of noise points due to erroneous triggering. On the contrary, if the threshold is defined to be larger (as indicated by the dotted line), it is able to ensure that data is not interfered by noises, but the linearity of raw data is lost. In FIG. 7, the determination of whether there is touch data depends only on the threshold. When the threshold is defined on the dotted line, the de-noise capability is high but the linearity is obviously sacrificed. On the other hand, when the threshold is defined on the solid line, the linearity is increased but the de-noise capability is decreased.
Furthermore, the aforementioned method may exhibit different linearity due to different electrode patterns on the touch panel. Some touch panel may have a specifically designed electrode pattern to increase the linearity and sensing value. However, not all touch control system manufacturers can adapt such a specifically designed electrode pattern and the problems still exist. Therefore, it is desired to provide a method for improving linearity of touch system coordinates.